Inclusive prompt photon production in nuclear collisions at RHIC and LHC

TL;DR

This paper addresses how inclusive prompt photon production in nuclear collisions can constrain gluon nuclear PDFs by performing NLO pQCD calculations with EPS09, nDS, and HKN07 nPDFs for $d$--Au at RHIC and $p$--Pb at the LHC. It emphasizes a factorized framework that combines direct and fragmentation photon contributions, uses standard PDFs and photon fragmentation functions, and systematically assesses scale uncertainties. The authors find notable differences among nPDF sets across rapidities and collision energies, with forward rapidities at the LHC particularly sensitive to small-$x$ gluon shadowing, and mid-rapidity measurements at RHIC probing anti-shadowing and EMC effects; EPS09 uncertainties are quantified via error sets. They also provide perturbative QCD baselines for $A$--$A$ collisions and explore the potential suppression of the fragmentation component due to medium effects, illustrating how isolated-photon measurements could help disentangle cold and hot nuclear matter influences on hard processes such as jets and high-$p_T$ hadrons.

Abstract

Nuclear modification factors of inclusive prompt photon production in d-Au collisions at RHIC and p-Pb collisions at the LHC are provided at different rapidities. The calculations are performed at NLO accuracy using the EPS09 NLO nuclear parton distribution functions (nPDFs) and their error sets. The results are compared to the ones obtained with the nDS and HKN07 NLO nPDFs, and to the corresponding nuclear modification factors of neutral pion production in these collisions. The sensitivity of these results to the scale choice is also investigated. Interestingly, the predictions using the different nPDF sets differ from each other to the extent that this observable can be expected to become very useful for probing nPDFs over a wide range of Bjorken-x. In order to obtain a perturbative QCD baseline in heavy-ion collisions, calculations are carried out for minimum bias Au-Au collisions at RHIC and Pb-Pb collisions at the LHC. We also estimate the maximal possible suppression which the produced QCD matter can be expected to have on inclusive prompt photon production due to the quenching of the fragmentation component. The nuclear modification factor for prompt photon production is thus suggested to be used for gauging both the cold and the hot nuclear matter effects on other hard processes which are expected to be affected by quark-gluon plasma formation, such as large-pT hadron and jet production.

Figures (10)

• Figure 1: The nuclear modifications of the gluon PDF in a Pb-nucleus at $Q^2 = 1.69$ GeV$^2$ (left) and $Q^2 = 100$ GeV$^2$ (right) in the EPS09 set (and their uncertainties), nDS set and HKN set.
• Figure 2: $d\sigma/dp_{_T} dy$ inclusive photon production $p_{_T}$-spectra in minimum-bias $d$--Au collisions at RHIC ($\sqrt{s_{_{\mathrm{NN}}}}=200$ GeV) and $p$--Pb collisions at LHC ($\sqrt{s_{_{\mathrm{NN}}}}=8.8$ TeV). The band (not visible at LHC energy) corresponds to a scale variation from $\mu=M=M_F=[p_{_T}/2;2p_{_T}]$.
• Figure 3: Nuclear modification ratio $R_{_{d {\rm Au}}}^{\gamma}$ of inclusive photon production at $|y|\le0.35$ in $d$--Au collisions at $\sqrt{s_{_{\mathrm{NN}}}}=200$ GeV using EPS09 nPDFs, in comparison to (i) nDS and HKN sets (left), and (ii) the pion case, $R_{_{d {\rm Au}}}^\pi$ (right).
• Figure 4: Same as Fig. \ref{['fig:photon_y00_0200']} at $y=3$.
• Figure 5: Left: Prompt photon quenching factor $R_{_{d {\rm Au}}}^\gamma$ as a function of $p_{_T}$ in different rapidities: $y=0$ (solid), $y=1.6$ (dotted) and $y=2.4$ (dash-dotted) with EPS09 nPDF corrections. Right: Double ratio $R^{\rm iso}_{_{d {\rm Au}}} \equiv R_{_{d {\rm Au}}}^\gamma({\rm EPS09})/R_{_{d {\rm Au}}}^\gamma({\rm proton\ PDFs})$.